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Cactus bugs compensate for lack of weapons with bigger balls

Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world

By Harry Pettit

Unarmed but ready for action

Christine Miller

Species: Leaf-footed cactus bugs (Narnia femorata)

Habitat: prickly pear cacti around the southern US and Central America

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Silver linings can be hard to come by, but for the leaf-footed cactus bug – who appear to grow larger testes in response to losing their sparring weapons – life could certainly be worse.

Leaf-footed cactus bugs – or Narnia femorata – generally grow to between 14 and 19 millimetres and live on prickly pear cacti around the southern US and Central America. The insects feed on the fruit and joints of the cacti on which they reside – and fight their existential battles.

Males compete over territory and mates by swivelling around and grappling one another using their barbed hind legs in an attempt to wrestle and kick their rival off the cactus pad arena.

Normally, when a wild animal loses its weapon the outcome is poor, if not fatal, as it can no longer compete for resources.

But a study by Paul Joseph and Christine Miller at the University of Florida in Gainesville suggests that for the leaf-footed cactus bug, disarmament leaves the insects far from helpless.

“Males may encounter problems that negatively affect weapon growth during development to adulthood, like self-amputating a limb as a self-defence mechanism to escape predators,” says Joseph. “We wanted to see if these males, who dropped a weapon, allocate more resources to another trait that increases reproductive success – testes.”

Joseph and Miller divided young males into control and weapon-less groups and allowed them to reach sexual maturity. They found that young males that dropped a weapon during development grew 20 per cent larger testes than the control groups.

“Larger testes create more sperm, so males with larger testes – but lacking a weapon – may be able to compensate by fertilising more eggs in the few mating opportunities they do achieve than males with intact weapons but smaller testes,” says Joseph. The team presented its findings at the 53rd Annual Conference of the Animal Behaviour Society in Missouri on 31 July.

The existence of such a trade-off is intriguing, says Clint Kelly at the University of Quebec in Montreal, Canada.

“If larger weaponry increases a male’s mating success, and larger testes increase a male’s fertilisation success, then why do males not invest more into each of these traits at the expense of some other non-sexually-selected trait, such as middle legs?” he asks.

“In all likelihood it does not pay for a male to increase investment in both traits concurrently, perhaps because males with larger weaponry face reduced levels of sperm competition and thus have no need to boost the size of their ejaculates, which are energetically expensive to produce.”

Kelly says the study will help identify whether this strategy only occurs in cactus bugs or “is a deeply rooted developmental strategy among animals”.